1. Field of the invention
The present invention concerns a device for eliminating low frequency interference noise from a transmission system, and in particular l/f noise within a homodyne radar receiver.
2. Description of the prior art
Within a homodyne receiver, the frequency change restores the base band, i.e. the intermediate frequency is zero. The application of homodyne receiving to Doppler pulse radars, for ground-to-air surveillance, intended to identify targets of very low radial velocity, requires a processing capacity which is practically homogeneous from the continuous frequency up to the repetition half-frequency of the emission signal. More especially, the constant detection probability processing of a given equivalent radar surface being able to be displaced at variable radial speed supposes the invariance of the signal/noise ratio of the target over the whole length of the spectrum to be analysed (above all, in the event where the processing of the signal received by the radar utilizes a spectrum analyser). The noise acting on the signal received is formed more specifically by the thermal noise of the receiver, which has hardly any dependance upon the frequency in the band frequency involved, and by the noise due to the semi-conductors utilized in the first stages of the receiver, this noise being inversely proportional to the frequency l/f such that this noise becomes equal to the thermal noise of the receiver at a frequency of several hundred Hertz and thereafter increases by 3 dB per decreasing octave, according to a well-known asymptotic curve of order 1. This noise in l/f is that much more likely to impair performances as it is standard to operate radar with a noise/signal ratio close to 0 dB. An increase of the noise in a given frequency range can thus mean the loss of echoes in this frequency range.
In order to achieve the said invariance of the noise signal ratio, it is necessary to eliminate the noise in l/f. A simple filtering of the disturbed frequency zone renders it possible to suppress the noise, thus not to provoke an inacceptable rise in the rate of false alarms, but the noise/signal ratio in this frequency range is not improved due to the fact the signal has simultaneously been attenuated, and consequently it is not possible to carry out a correct analysis of the useful signal.
The object of present invention is a circuit that is easy to produce and which allows simply and inexpensively to suppress practically all the low frequency noise present in transmitting-receiving system, between the beginning of the amplifier stages of the transmitter and the final amplification stages of the receiver.